a) Field of the Invention
The present invention relates to a semiconductor laser, and more particularly to a semiconductor laser having lattice mismatch between its substrate and clad layer.
b) Description of the Related Art
A conventional semiconductor laser of 1 .mu.m band uses typically an InP substrate and a lamination formed thereon and made of InGaAsP based materials which lattice match the substrate. A heterojunction made of InGaAsP based material which lattice matches InP is difficult to form a sufficiently large band energy gap. Semiconductor lasers having such a heterojunction have the characteristics (threshold value, efficiency, and etc.) greatly dependent upon temperature.
Group III-V semiconductor having the same crystal structure has generally a tendency that the smaller the lattice constant is, the larger the band energy gap and the smaller the refractive index. A difference between band gaps is mainly a difference between energy levels of conduction band edges.
The present inventor and his colleagues have proposed a semiconductor laser including a substrate with a smaller lattice constant and a larger band gap than InP, a clad layer, an active layer, and the like laminated on the substrate through a buffer layer, wherein the buffer layer functions to relax lattice mismatch, and the active layer constitutes a strained quantum well layer (Japanese Patent Laid-open Publication No. 6-326407).
This semiconductor laser uses GaAs as the material of a substrate, a composition graded layer of InGaAs changing its In composition continuously from 0 to 0.3 is formed on the substrate, and a lamination of semiconductor layers of a laser structure is formed on the composition graded layer. The semiconductor layers other than the active layer are made of InGaP, InGaAsP, or InGaAs material which lattice matches In.sub.0.3 Ga.sub.0.7 As. The active layer is In.sub.0.4 Ga.sub.0.6 As. Light confining layers of In.sub.0.4 Ga.sub.0.6 As.sub.0.8 P.sub.0.2 are formed on both sides of the active layer. Clad layers of In.sub.0.8 Ga.sub.0.2 P are formed on opposite sides of the light confining layers. The In.sub.0.4 Ga.sub.0.6 As active layer and light confining layers on both sides thereof constitute a strained quantum well structure of separate confinement hetero (SCH) structure.
It is possible for the light confining layers to use InGaAsP compositions having a larger energy level of the conduction band edge and a wider forbidden band than InGaAsP composition which lattice matches InP. Leakage of charge carriers was reduced and the temperature characteristic was improved. For example, a characteristic temperature of about 100 K was achieved.